This invention is generally related to the field of telecommunications devices, such as telephones, facsimile machines, computer modems, automatic alarm or diagnostic dialers, etc., which operate over telephone lines. More particularly, this invention is related to a circuit and method that may be used in such devices for determining the connection status of a telephone line shared by the device, e.g., determining whether the telephone line is On-Hook or Off-Hook.
It is typical that a residential or business user may want to connect more than one telecommunication device in one common telephone line. When one of the devices has an auto-dialing function, such as in a facsimile machine or a modem, conflicts may arise if the telephone line is in use (Off-Hook) when the automatic device attempts to dial a number. The presence of the automatic devices should not impact on normal usage of the telephone line for voice communications. Therefore, the devices should have some means of determining when the telephone line is being used for normal voice communication on one of the local telephones. Further, the devices should not attempt to use the telephone line if it can be determined that the line is in use, and such determination should be made without interfering with existing communications. Similarly, when the automatic device is using the telephone line, it should abandon the line if one of the local telephones goes Off-Hook. Further, the automatic device should not prevent normal usage of the telephone line in an emergency situation. For the various reasons given above, the devices should have circuitry for detecting whether the telephone line is Off-Hook or On-Hook during various operational modes such as before dialing is attempted, or for determining whether a present communication should be temporarily discontinued.
As will be appreciated by those skilled in the art, when a telephone line is in On-Hook condition, i.e., when the telephone handset is resting on its cradle, there is virtually no current flow, and the line voltage is relatively high, typically 50 V DC. Conversely, when a telephone line is in Off-Hook condition, the phone circuits complete a circuit loop and there may be current consumption of, for example, 10-50 mA DC, and the line voltage is low, about 1-10 V DC. The line state may thus be detected either by sensing the current in the line, or by sensing the line voltage.
Generally such telecommunication devices include a line interface between the telephone line and their data circuits and that line interface is chosen to provide a suitable impedance match between the telephone line and the data circuits in the device. Further, such line interface is required to galvanically isolate the data circuits from the telephone line. The galvanic isolation is typically achieved by the use of transformers, opto-couplers, relays, or by a combination of such components. Thus, the data circuits may be electrically isolated from the telephone line while the signal information is electromagnetically or optically coupled therebetween depending, for example, on whether a transformer or an opto-coupler is respectively used to provide the galvanic isolation.
Although determining whether a respective device is loading the telephone line is conceptually relatively straight forward, such as could be made by tracking DC voltage measurements between respective ring and tip lines of the telephone line. Unfortunately, in practice such determination is not so simple. One problem is that the characteristics of the telephone line itself can greatly change, both from site to site and at a single site over a period of time. Further, as suggested above, when the device is using the telephone line it should be able to detect when one of the local telephones goes off hook. In this case, making an accurate determination becomes problematic since there may be great variation in the line loading created by different types of telephones. Some prior art line connection detectors, have used opto-coupler devices for sensing line current variation. Unfortunately, such detectors are prone to errors since they are limited to operate in a relatively narrow range where they exhibit linear characteristics. Other prior art devices have required use of analog-to-digital converters which add to the cost and complexity of the detector.
From the foregoing it will be appreciated that it is desirable to provide a circuit and method that may be used by the telecommunication devices for determining the connection status of the telephone line that has the following characteristics: maintains substantial isolation from the line, responds promptly and accurately even in the presence of noise and conversely does not generate noise which can interfere with data/and or voice transmission, requires no interaction between local and remote locations to function, is impervious to the non-ideal characteristics of real circuit components, such as the presence of offset voltages, bias currents, etc., is inexpensive and reliable.
Generally speaking, the present invention fulfills the forgoing needs by providing a circuit for determining connection status of a phone line including respective tip and ring lines. The circuit comprises a first operational amplifier coupled to receive a signal indicative of a voltage difference between the respective tip and ring lines. The circuit further comprises a lag network coupled to impart a predetermined delay to the output signal from the first operational amplifier. A second operational amplifier is coupled to receive the voltage difference signal. The output signal from the second operational amplifier has a sufficiently fast time response relative to the output signal from the lag network.
The present invention further fulfills the forgoing needs by providing a telecommunications device, such as a modem, having a circuit for determining connection status of a phone line shared by that device with a phone. The phone line includes respective tip and ring lines. The device comprises a controller coupled to receive from the circuit a signal indicative of the connection status of the phone line and having means for initiating or interrupting a respective communication based on the indicated line connection status, and wherein the circuit in turn comprises a first operational amplifier coupled to receive a signal indicative of a voltage difference between the respective tip and ring lines. The circuit further comprises a lag network coupled to impart a predetermined delay to the output signal from the first operational amplifier. A second operational amplifier is coupled to receive the voltage difference signal, the output signal from the second operational amplifier having a sufficiently fast time response relative to the output signal from the lag network.
In another aspect of the invention, the foregoing needs are further fulfilled by providing a method for determining connection status of a phone line including respective tip and ring lines. The method allows for receiving in two circuit paths a signal indicative of a voltage difference between the respective tip and ring lines. The method further allows for imparting in one of the paths a predetermined lag to the signal propagating therethrough so that the signal propagating in the other path has a sufficiently fast time response relative to the signal propagating in the one path with the lag.